1. Field of the Invention
The present invention concerns a method of and an apparatus for forming highly oxidative water (water containing ozone and/or OH radicals at high concentration), for use in purification of water of lakes, marshes and rivers or contaminated water containing organic or inorganic deleterious materials such as industrial waste water, particularly, in toxifyng treatment of contaminated water containing less biodegradable deleterious organic materials, particularly, dioxin or the like, or in cleaning, resist defoliation and formation of oxide layers in the production of semiconductors.
The less biodegradable organic materials include, for example, natural or synthetic polymeric compounds such as celluloses, humines materials, surfactants, dyes, rubbers and resins; aromatic compounds such as benzene, toluene, xylene and phenol; aldehyde compounds such as acetaldehyde and crotone aldehyde, and oils and fats, higher fatty acids and other COD ingredients. Concrete example of waste water containing such deleterious organic materials include, for example, waste water from chemical plants, waste water from medicine factories, waste water from foodstuff factories, waste water from oils and fats factories, waste water from pulp factories, or like other industrial waste water, water of rivers and water of lake and marshes.
Further, in the semiconductor production processes, utilization is expected for as cleaning water in place of hydrofluoric acid cleaning, oxide layer forming agents in place of dense oxide layer formation using an oxidation furnace, and defoliating agents in place of resist defoliating agents using sulfuric acid or deleterious organic solvents.
2. Description of the Related Art
Biological treatment of waste water containing organic materials (water to be treated) under the presence of active sludges containing aerophilic or aerophobic microorganisms has been conducted generally. However, the biological treatment, if applied alone, requires a long time for the treatment in a case where less biodegradable materials such as polymeric materials, aromatic compounds and COD ingredients are contained in the waste water, failing to obtain a satisfactory cleaning effect. Further, if the quality of water to be treated fluctuates, the quality of the resultant purified water also fluctuates, failing to obtain cleaned water of stable cleanness.
In view of the above, when waste water containing less biodegradable materials is treated, a method of forming aqueous ozone by aerating and stirring gaseous ozone and transforming the materials described above into biodegradable materials by utilizing the oxidative activity of ozone is adapted. Known methods of obtaining gaseous ozone include a method of applying a high voltage from electrodes on both side ends of a air feed pipe through which air or oxygen rich air is sent, to conduct discharge and transform oxygen into ozone.
However, this method is extremely poor in the electric power efficiency, the gaseous ozone is instable of and tends to be decomposed thermally in contact with the wall surface upon pressurization, transportation or dissolution in water, so that the efficiency for the entire equipment is poor and lack in practicality.
For overcoming the problem in the prior art, Japanese Patent Publication No. Hei 5-87320 discloses a method of conducting discharge in waste water containing COD ingredients, thereby forming ozone or active oxygen species (OH radical) directly from dissolved air (oxygen) in water, or generating UV-rays as an accelerator to increase BOD/COD ratio, transforming the same into a biodegradable waste water and then applying biological treatment. However, since it is actually difficult to attain stable discharge in water over a wide area, it has not yet been put to practical use.
One of the methods of attaining the wide area discharge is discharge that utilizes high velocity pulses as described in Japanese Patent Laid-Open Hei 9-299785. Further, since it has been found that discharge in water proceeds with fine bubbles present to some extent in water being as nuclei, Japanese Patent Laid-Open Hei 5-319807 proposed a method of forming fine bubbles by positively and efficiently aerating air or oxygen between electrodes.
However, a technique capable of ensuring oxidative performance at a practical level by discharge in water (ozone and/or OH radical concentration) under stable power efficiency and productivity has not yet been established at present.
Further, in a case of discharge in water, since electrodes immersed water as an object to be processed are always exposed to a highly oxidative material (ozone or OH radicals) thus formed, it involves a problem that the surface of the electrodes is corroded. Further, upon treating waste water containing organic materials, particularly, in a case where aromatic compounds such as benzene, toluene, xylene and phenol are contained, they are decomposed as far as organic acids such as oxalic acid or formic acid to form acids, so that metal electrodes in direct contact with water are corroded remarkably to result in a significant problem in view of practical use.
Further, in a semiconductor application use is in which ultrapure water is used as water, since leaching of metal electrodes caused by the oxidative activity of water containing ozone or OH radicals (pollution) causes a significant problem, aqueous ozone for the semiconductor use is formed at present imitatively by an aeration/dissolution system of gaseous ozone and since this imposes a limit for increasing the ozone concentration, and gives a hindrance for putting into practical use as the highly oxidative water in place of existent oxidative chemicals.
For directly forming a highly oxidative water by discharge in water, it is important to attain discharge in water over a wide area and stably. For this purpose, it is important, for example, that fine bubbles of uniform size as nuclei for discharge are formed uniformly and efficiently over the entire discharge space, the bubbles themselves are mainly composed of oxygen as an oxidative source, a high voltage electric field has a function not concentrating locally but uniformly extending over the entire region of the discharge space, discharge is not joined and concentrated and that there is no problem caused by leaching of metal electrodes.